Various products are fabricated in a continuous production line by the sequential addition of components to previously supplied components. This is particularly advantageous when one or more of the components can be supplied in the form of a continuous sheet or web. Thus, in the formation of disposable absorbent articles such as diapers, incontinence devices, sanitary napkins, and the like, the liquid impermeable outer layer is normally supplied at a point in the fabrication line in the form of a continuous roll of plastic film and the liquid permeable inner liner can be supplied at another point in the fabrication line in the form of a continuous roll of non-woven fabric, while absorbent pads, waist elastic bands, leg elastic bands, and/or other elements can be supplied at different points in the fabrication line as discrete objects. While the outer layer and the inner liner can, but need not, be coextensive in their dimensions, both in the machine direction and in the transverse direction, the dimensions of most if not all of the other components represent only a fraction of the dimensions of the outer layer. Thus, it is necessary to provide some means for bringing the various components of a single composite product together so that the components in the composite product are in proper registration with respect to each other. However, variations in the components or in the operating environment can cause components to be mispositioned or even omitted from a particular composite product. Mispositioned or missing elements can degrade the quality of the composite product or even render the composite product unsuitable for its intended purpose.
While the in-process products can be inspected manually or automatically at various stages along the fabrication line, an ineffective or inaccurate inspection may not maintain the composite product within its design specifications and/or may not efficiently and accurately cull out the unacceptable composite products. Unnecessary shut-downs of the fabrication line reduce production efficiency and increase product costs.
Various methods and apparatuses have been developed for the purpose of inspecting composite products which are fabricated from at least one moving web. For example, U.S. Pat. No. 4,837,715 to Ungpiyakul et al., issued Jun. 6, 1989 to Kimberly-Clark Corporation, discloses an apparatus for determining the relative locations of selected components within a product segment of a moving web. Photoelectric detectors can be mounted at a plurality of locations along the machine direction length of the composite product fabrication equipment whereby the relative locations of components in the machine direction can be determined. Optical brighteners can be incorporated into various components of the composite product being formed from a web, in order to facilitate detection of selected components utilizing an ultraviolet light source. A plurality of photoelectric detectors can be disposed serially along the machine direction in order to detect the location of different selected elements, and the resulting output signals can be stored for subsequent association with other measurement signals for the same individual product.
U.S. Pat. No. 4,680,205 to Lerner et al., issued Jul. 14, 1987, discloses the use of electromagnetic radiation shifting indicia incorporated into a component of a composite being formed from a web in order to facilitate detection of the component. If such component is covered up during the fabrication process, an additional component having the radiation shifting indicia can be added to the product. Only those elements of the web in which the shifting indicia is incorporated will be activated by ultraviolet light to produce visible light. The presence or absence of the thus produced visible light can be detected by photoelectric detectors as an indication of the presence and position of the selected component. A series of photoelectric detectors can be disposed serially along the machine direction in order to detect the location of the sequentially selected components.
While such inspections systems may be satisfactory for some installations, they are particularly deficient when it is desirable to determine the registration of one or more components in the transverse direction instead of making all of the measurements in the machine direction. The machine direction is the direction in which the in-process articles move along the fabrication line, while the transverse direction is perpendicular to the machine direction. Thus, for a continuous web component, the length of the web would be in the machine direction, while the width of the web would be in the transverse direction. Such conventional apparatuses do not provide for the inspection of the registration of web components based upon their relative positions transverse to the machine direction. That is, such conventional apparatuses are able to measure the machine direction spacing between selected web components but not the transverse direction spacing.
Another problem is encountered with registration detection systems employing a plurality of sensors which are located at various points along the length of the fabrication equipment. In many composite articles produced from at least one continuous web, the continuous web component is thin and subject to stretching. Since it is virtually impossible to maintain the tension of the continuous web component constant during the various fabrication steps, the degree of stretching that the continuous web component undergoes can vary. Such stretching can cause variations in the registration of the initially deposited components with respect to subsequently deposited components or subsequent processing operations. Thus, even though an early component may be initially within its acceptable position range, the stretching of the continuous web or the jostling of the discrete components during the fabrication process may result in the early component being outside of its acceptable position range in the final composite product.
Accordingly, it is an object of the invention to provide a method and apparatus which is capable of measuring the relative positions of components of a composite article in the transverse direction or in both the machine direction and the transverse direction. It is an object of the invention to provide a more complete inspection of the positional relationships of components in a composite article, particularly in such an article produced from at least one continuous web. It is an object of the present invention to conduct, at a single location in the fabrication process, an inspection of the positional relationship of components in a composite article. It is another object of the invention to perform rapid analyses of web sections as they pass along the machine direction of the fabrication equipment. A further object of the invention is to provide for feedback control of the operation of the fabrication equipment to maintain the product specifications within acceptable ranges. An additional object of the invention is to provide improved means to cull any defective products. Another object of the present invention is to at least reduce, if not eliminate, the need for human monitoring and intervention in the fabrication process.